brass pins, termolock or the four square pins that come with the grs 30 vise attachments set the one with a half round channel work very well, now i'm using a prototype of universal coin holder made by Lyndsay and is great , allow you to hold all type of coins but is not for sale yet.
I'd asked Lindsay about this a while backwhen I ordered my nickel holder, and there was no interest in them.
These are what I came up with. A set of different size rings to stack to support coin underneath, and clamp tightly around the rim without getting in the way, or leaving marks on the coin.
I just can say that lately since i have start carving and sell Clad Coins ( CCC , Clad Cameo Carvings) the attention of hobo nickels carvers to carve other type of coins that are not just nickels grow very much and now have become a sort of new trend .
1 years ago maybe was not very common see other type of coins carved but now yes.
BPE Engraving has a really nice little nickel holder made out of carbon fiber. I've made similar style ones with Thermoloc for pennies and quarters (and one for nickels to use with my micro-block that doesn't have the notch the BPE needs for centering). It only takes two sticks of Thermoloc to make a coin holder. Actually you should have a small blob left over.
The key is to either have the GRS Thermoloc fixture making block or to simulate it - you want to make a hinged piece so you can put good pressure on the coin using your vise jaws.
Any notions of when the universal holder will be available? Looks interesting. Otherwise, I use the nickel holder for nickels, thermoloc or the pins for other sizes.